Broad-beam mass spectrometer having particle energy selection means



Aug. 13, 1968 J. M. SAARI ETAL 3,397,311

BROAD-BEAM MASS SPECTROMETER HAVING PARTICLE ENERGY SELECTION MEANSFiled Feb. 12, 1965 3 Sheets-Sheet 1 INVENTOR. JOHN M. SAAR/ fi/C'HAFDI. SCHOEN IRVING E. DAYTON ATTORNE Y Aug. 13, 1968 J. M. SAARI ETALBROAD-BEAM MASS SPECTROMETER HAVING PARTICLE ENERGY SELECTION MEANSFiled Feb. 12, 1965 5 Shets-Sheet 2 PULSE FORM/N6 C/RC U l T arm 26.

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REPELLEK gAGE JOHN M. 54AM RICHARD I. SCHOEN \RVNB; E. DAYTON ATTORNEYJ. M. SAARI ETAL BROAD-BEAM MASS SPECTROMETER HAVING PARTICLE ENERGYSELECTION MEANS Filed Feb. 12, 1965 5 Sheets-Sheet 5 F n fv n va 2CIRCUIT 4/ POWER SUPPLY INVENTOR. JOHN M. SAARI RICHARD I. SCHOEN\RVINGL E. DAY TON 4TTOR/VEY Uni v 3,397,311 BROAD-BEAM MASSSPECTROMETER HAVING PARTEQLE ENERGY SELECTEON MEANS John M. Eiaari,Mercer island, and Richard L. Schoen,

Kent, Wash, and Irving E. Dayton, Bozeman, Mont.,

assignors to The Boeing Company, Seattle, Wash, a

corporation of Delaware Filed Feb. 12, 1965, Ser. No. 432,244 3 Claims.(Cl. 250-413) ABSTRACT OF THE DISCLOSURE This invention relates toapparatus for the analysis of molecules present in an unknown mixture.More particularly, the instant invention relates to apparatus forseparating ions having differing masses by the use of electrostatictechniques so as to provide a determination of the masses of thedifferent ions and to thereby identify the different ions.

It is well known in the mass spectrometer art to analyze an unknownmixture of molecules by imparting velocity to ions of the differingmolecules and subjecting the ions to a magnetic field which causes theions to deviate from a linear path. The amount of deviation each ionexperiences as a result of the applied force field is directly dependentupon the mass of the individual ion; the ions of different mass becomeseparated as they travel through a relatively great distance. Bycollecting the ions and measuring the amount of deviation of each groupof unknown ions, from the direction of initial velocity, the mass of theions in the unknown group can be determined.

An example of the mass spectrometer art is presented in United StatesPatent No. 2,774,882 by William H. Wells, assignor to Bendix AviationCorporation. As taught in the Wells patent, mass spectrometers whichemploy a magnet to produce deviation of moving ions are not entirelydesirable for several reasons. For example, relatively large mganetsmust be used, necessitating apparatus which is fairly heavy, bulky andrelatively expensive. Furthermore, as taught in the Wells patent,spatial dispersion of ions due to a magnetic force field does not have alinear relationship with respect to the mass of the ions, so thatspecial circuits or additional computations are required to determinethe ions of different mass in an unknown mixture. In the prior art, useof DC. amplifiers to amplify the output signals caused by ionimpingement upon collector plates is not desirable since such amplifierscannot provide the sensitive response which is characteristic of AC.amplifiers.

As taught in the Wells patent, it is desirable to construct massspectrometer apparatus having means for applying a force on a pulse ofions for a predetermined period of time in a first direction so as toimpose a constant momentum to the ions. Such a mass spectrometerpreferably includes apparatus for imposing an electrostatic field on thepulse of ions in a second direction; as taught in the Wells patent andin the general mass spectrometer art, this second direction issubstantially perpendicular ed States Patent 3,397,311 Patented Aug. 13,1968 to the first direction so as to produce a linear separation of theions in the second direction on the basis of their mass. As will betaught below, the instant invention provides an improvement over theWells patent and other mass spectrometer patents by imposing at leastone electrostatic field on the pulse of ions in a second directionparallel and opposite to the first direction. In mass spectrometer art,as exemplified by the Wells patent, ions are detected by theirdisplacement in a second direction, i.e., transverse to a firstdirection, after they have traveled a relatively great distance in thefirst direction. Since the spatial dispersion of the ions in the seconddirection is linearly related to the masses of the different ions, asimple determination of ion masses can be made by comparing the spatialdispersion acquired with the unknown ions with the tabulated andrecorded dispersion of known ions. As taught in the Wells patent, thisdetermination is enhanced by the use of AC. amplifiers which areemployed and are resonant at the repetition rate for producing the ionpulses.

The instant invention proposes apparatus for the determination anddiscrimination of the masses of an unknown ionized mixture of moleculeswhich is not limited to a narrow beam of ions, moving in a firstdirection, as the Wells patent is limited. By comparing the method ofselecting ion masses according to the teachings of the instantinvention, which depends upon energy of ions all having constantmomentum, with the aforementioned \Vells method, we note in particularthat the instant invention is not limited by slits in portions of theapparatus which restrict ion beam width. Therefore, in the instantinvention, the ionized beam may be wide initially or may be fairlynarrow initially and may be allowed to expand in width in practice ofthe method according to the instant invention. Furthermore, theresolution of the instant apparatus does not depend upon the width ofthe ion beam. In prior art spectrometers the ion beam cannot be allowedto expand because of loss of resolution caused by the finite width ofthe beam compared to the Width of the opening through which the ion beamis discharged toward a collector plate. This limitation does not applyto the instant invention.

There is a second feature which distinguishes the instant invention overthe prior art, and, in particular, the patent to Wells, in addition tothe feature noted above, i.e., the capability of the instant inventionto handle an ion beam with minimum dimensional restriction. Inparticular, the second feature of the instant invention provides formass separation of ions in a second direction on a nonlinear basis,rather than a linear separation of ions in a second direction withrespect to the first direction given to the ions at the time velocity isimparted to them. Thus, the instant invention can be used to advantagein certain situations in which the prior art cannot be used to suchadvantage. For example, it may desirable during ion separation anddifferentiation to allow ion molecule reactions to take place. In such asituation according to the teachings of the instant invention, therewill be a controlled variable pause between the end of the ionizingpulse and the initiation of the velocity imparting repeller voltagepulse. During this pause, ion molecule reactions will be allowed to takeplace and the beam will expand; that is, ions will move around, and theion beam will increase in width. Another example involves a situation ofmonomolecular decomposition in which situation, as in the above notedexample of ion molecule reactions, the ions would be caused to lie or tobe situated within a chamber where ionization is caused between arepeller plate which imparts velocity to the ions in a first directionand a first grid or force field parallel to and opposite to the ion beammoving in a first direction for a period of time which 3 would be variedaccording to conditions under the particular experiment.

An object of this invention is to provide apparatus for determining themasses of ions in an unknown mixture including means for imposingsubstantially the same momentum on all of the ions and means forselecting those ions whose energies lie within a narrow predeterminedrange.

A second object of the instant invention is to incorporate relativelyinexpensive and compact features into a mass spectrometer through theuse of an electrostatic field to produce a separation of ions on thebasis of mass.

A third object of the instant invention is to utilize AC. or DC.amplifiers in mass spectrometer apparatus in order to obtain adetermination of the masses of the different ions in an unknown mixtureand to provide a sensitive and reliable indication of the said ionmasses.

A still further object of the instant invention is to provide a massspectrometer capable of separating ions in a beam of greater size thanheretofore possible with known apparatus.

Still another object of the instant invention is to provide apparatusfor a mass spectrometer wherein analysis of pulsed ions can be madewhile allowing ion molecule reactions to take place.

A still further object of the instant invention is to provide ion massanalysis using mass spectrometer apparatus wherein ions are analyzedwhile allowing for monomolecular decomposition to occur.

Briefly, the instant invention comprises means for providing pulses ofions, means for imposing a force on the ions in a first direction for apredetermined period of time, means for imposing a force on the ions ina direction substantially parallel and opposite to the first directionto provide a selection of the ions on the basis of their energy. Theselection of the ions on the basis of energy can be related to the massof the ions in the following manner: the well known equation for kineticenergy may be written in terms of momentum as:

in which M is the mass of each individual ion, v the velocity of eachindividual ion, and P the momentum of each individual ion. Since by theteachings of this invention each individual ion is given substantiallythe same momentum as every other ion, it is readily seen from Equation 1that selection of the ions on the basis of energy produces massselection.

Other objects and advantages will be apparent from the detaileddescription of the invention which follows and from the appendeddrawings and claims.

In the drawings:

FIGURE 1 is an isometric perspective illustrating an embodiment of theinstant invention;

FIGURE 2A is a cross-sectional view through the midplane of theembodiment of FIGURE 1;

FIGURE 2B is a diagram illustrating time versus intensity of ionizingbeam and repeller voltage pulses; and

FIGURE 3 is a view in cross-section showing a modification of theapparatus shown in FIGURE 1 and FIG- URE 2A.

Referring to FIGURE 1, a mass spectrometer according to the teachings ofthis invention is shown. A partial vacuum bell-jar 19 (not shown inFIGURE 2A and FIGURE 3) is provided having bolt fastening means 34 for aremovable cover plate 12; the cover plate 12 including means defining ascalable opening 13 to permit passage of gas molecules into bell-jar 19from an exterior container (not shown). The bell-jar 19 shown in FIG-URE 1 encloses: means for providing pulses of ions within the bell-jar19 including an electron gun 17 disposed to be connected through asealed opening 32 in plate 12 to a first pulse forming circuit 41; meansfor imposing a force on the ions in a first direction for apredetermined period of time including a voltage repeller plate 15supported on plate 12 by insulating members 14, and connected by a leadthrough a hole 32 in plate 12 to a second pulse forming circuit 40,which in turn is connected by delay circuit means 42 with the firstpulse forming circuit 41; and means to provide a selection of the ionson the basis of their energy including means for imposing a force on theions in a direction substantially parallel and opposite to the firstdirection, at least one means for reflecting ions having energies withina narrow predetermined range, means for collecting the ions andindicator means for indicating the current imposed by the ionimpingement upon said means for collecting the ions.

As seen in FIGURE 1 and FIGURE 2A, the means imposing a force on theions in a direction substantially parallel and opposite to the firstdirection includes a grid system 20 having a first grid 20a acquiring apotential from a power source 8 and a second grid 20b which is grounded;the means for reflecting ions includes a grid system 26 having a firstgrid 26a which is grounded and insulated by supports 14a from a secondgrid 26b which is given a predetermined potential from a power source(not shown); the means for collecting the ions includes an ion collectorplate 30 (having support members not shown). The indicator meansincludes an electrometer 32a for indicating the current by ionimpingement on plate 30, and in the embodiment of FIGURE 1 and FIGURE 2Aa DC. or an AC. amplifier 32, to amplify the current at plate 30 due toion impingement.

In combination with the means described above and similarly enclosedwithin bell-jar 19 are: the plurality of insulating members 14 disposedfor attachment to plate 12 and supporting in conjunction with supportingbolts 24 a system of uniform plates or discs 16. The discs 16 arenumbered 1 through 11, disposed substantially parallel to one another,and the distances separating discs 1 through 9 and discs 10 and 11 areuniform. The discs 16 are connected through a number of resistance leadsto the second pulse circuit 40 in such a way as to maintain a uniformvoltage gradient, between any two of the plates 16, at a time coincidingwith the voltage on plate 15. In their central portion the discs 16collectively include means defining a chamber 18 within bell-jar 12within which the gas coming through opening 13 is ionized andaccelerated by plate 15. The grid system 20 is disposed in combinationwith the discs 16, substantially parallel thereto. A structural member22 is disposed substantially parallel to the plates 16 and is securedtherein by the bolts 24. As seen in FIGURES 1 and 2A, a second gridsystem 26 is supported upon member 22 by members 28 and 29. The system26 is disposed transverse to the plane of the member 22 and includes theparallel grids 26a and 26b separated by the insulators 14a. In FIGURE 2Athe ion collection plate 30 is disposed substantially perpendicular tothe plane of the member 22 by support means (not shown). It is to beunderstood, however, that collection plate 30 is not restricted to itsposition relative member 22 as shown in FIGURES 1 and 2A but may bedisposed at any angle to the plane of member 22. The cover plate 12includes means defining a plurality of seala'ble openings 32 throughwhich resistance leads are passed for interconnecting the plates 16 withthe second pulse circuit 40, and the grid 20a of FIGURE 3 and grids 20aand 26b of FIGURE 2A with the power source 8. Referring to theembodiment shown in FIGURE 3, it is readily seen that the means forreflecting ions having energies within a narrow predetermined range andthe accompanying supporting structure thereto, as described above in thedescription of the embodiment of FIGURE 1 and FIGURE 2A, are notincluded in the embodiment of FIGURE 3.

As seen in FIGURES 1, 2A and 3, ion-producing electron gun 17 isdisposed between the second and third discs of discs 16. The grid 20a,is disposed between and substantially parallel to discs 9 and 10.Disposed above disc 11 and substantially parallel thereto is grid 20b.

As seen in FIGURE 2A, the gun 17 and the repeller plate 15 are connectedto ground through resistors 36 and 38, respectively, so as to besubstantially at ground potential in the steady state operation. In thesteady state operation ionization is created by the ionizing beam fromelectron gun 17 represented by a dotted line between discs or plates 2and 3. This ionizing beam from electron gun 17 may be a beam ofelectrons; or the means of ionization may include instead of gun 17other charged particles, high speed uncharged particles, or a beam ofphotons. According to the instant embodiment the beam emitted fromelectron gun 17 is a pulsed beam derived through pulses of voltage ofsubstantially equal magnitude applied from a pulse-forming circuit 41.The delay circuit 42 provides means for varying the time between theproduction of ions by gun 17 and the imposition of the acceleratingforce to the ions, by the potential pulsed to repeller plate 15. Thisvariable pause controlled by circuit 42 allows time for ion moleculereactions to take place resulting in ion beam expansion. In the diagramof FIGURE 23 there is illustrated the ionizing sequence of the instantinvention with a time versus intensity diagram. First, the ionizingpulse from electron gun 17 is initiated at a time t and it is turned offat a short time later at t +r During their travel through the space orregion occupying chamber 18, the electrons from electron gun 17 impingeupon molecules of gas and vapor introduced into chamber 18 from acontainer (not shown) exterior to bell-jar 19 and entering chamber 18through opening 13 in the bell-jar cover plate 12. This causes themolecules in chamber 18 to be ionized into electrons and positive ions,most of which have a unitary charge.

By interrupting the electron stream coming from electron gun 17, theions are made available for easy impartation of velocity thereto uponthe imposition of a positive voltage pulse on the repeller plate 15.This voltage pulse is applied to plate 15 from the second pulseformingcircuit 40. As seen in the diagram of FIGURE 2B, this pulse is appliedat the time t +'r This pulse, as noted above, causes the ions to bedriven through chamber 18 by the repeller plate 15 toward the gridsystems 20 and 26 of FIGURE 1 and FIGURE 2A and toward the grid system20 of FIGURE 3. The pulse is applied to the plate 15 for a relativelyshort time, such as one or two microseconds, and is cut off. While theionizing beam from electron gun 17 is pulsed on and at all other timesexcept when the repeller voltage is on plate 15, the repeller plate 15voltage is the same as the voltage applied to the first grid 20a of gridsystem 20. The voltage on grid 20a, when dealing with positive ions, isat some negative value -V. The voltage on grid 20b of grid system 20 atthis time is zero, grid 20b being grounded; the voltage on grid 26a ofsystem 26 is zero, grid 26a being grounded; and the voltage on grid 26bof grid system 26 is at some value +AV, a small value. These grid systemvoltages are produced through resistors to the power supply 8 asindicated above. At this time, ion collection plate 30 is groundedthrough an electrometer 32a. The repeller voltage on plate 15 is pulsedon after the ionizing pulse on filament 17 is turned off. That is, therepeller plate 15 voltage from the second pulse circuit is turned on fora time which is short (r 1- compared with the transit time of the ionsto grid 20a. This repeller voltage pulse is indicated in the diagram ofFIGURE 2B. Because the voltage on grid 20a is maintained at a potentialV, there is an electric field generated between the repeller plate 15and grid 20a. This field is made more uniform by the existence of platesor discs 1 through 9, connected through resistors to the second pulsecircuit 40. The discs 1 through 9 have a uniformly descreasing potentialon each disc, i.e., disc 1 is at a higher potential than disc 2, etc.Thus a uniform electric field gradient is maintained between repellerplate 15 and grid 20a of FIGURES 1, 2A and 3. The electric field,uniform along the path the ions move in the first direction, effects thesame force on each ion thus giving each ion the same momentum. When therepeller plate 15 and discs 1 through 9 are pulsed to a positive voltageby circuit 40 driving positive ions toward grid 20a, constant momentumis acquired by all ions in chamber 1 8 having the same charge andprovided only that the time during which the pulse from circuit 40 isturned on is short compared to the transit time of the ions. As seen inthe diagram of FIGURE 2B, this time is (T2T1). Thus, all ions reach grid20a with substantially the same momentum. At this time there is avoltage difference between grid 20a and 2011, since 20b is grounded andgrid 20a has a potential V. The plates 10 and 11, which as noted abovein part form the plate system 16 are used to maintain this potentialgradient uniform. Thus as the ions enter the region bet-ween grid 20aand 2% there is a force caused by the uniform electric field due to gridsystem 20 and plates 10 and 11 which retards the ions. This force is inthe direction opposite to the direction in which the ions are moving. Ifthe ions have an energy which is sufiicient to overcome the potentialdifference V between grids 20a and 2012, then they pass into theequipotential regions between grid 201) and grid 26a. If these ions donot have such energy then they are reflected back into the chamber 18 bythe force due to potential difference V. Ions which have the necessaryenergy or more, that is, an energy equal to the charge of the electrontimes the potential V or more, will pass through grid 20b (a highlytransparent grid, as is grid 20a) and approach grid 26a. If these ionshave enough additional energy, they surmount the potential barrierbetween grids 26a and 261) which amounts to AV, a small amount (a fewvolts) as compared with several hundred volts, perhaps, between grids29b and 20a. If these ions have the additional energy to surmount thepotential difference between grids 26a and 26b they will passtherethrough. If they have less energy than is necessary to traverse theregion between grids 26a and 2612, they will be reflected as light wavesagainst a mirror from this region toward the collection plate 30 andwill be collected there. The current developing at collector plate 30from ion impingement is measured with an electrometer 32a. In thesteady-state operation which has been described, the measured current atthe collecting plate 30 is a pulsed current which may be measured by DC.or A.C. methods.

By altering the instant apparatus, as seen in FIGURE 3, it is feasibleand often desirable to apply to grid 20a a negative voltage V plus avoltage V sin wt which is a small A.C. voltage derived from power source8. Grid 20b is maintained at zero potential as in FIGURE 2A. Thisapplied modulated voltage will cause the ions to produce an alternatingcurrent voltage on the collection plate 30 of frequency w/ 211-. Themeasurements will be made with an alternating current apparatusamplifier 33 and an electrometer 32a as seen in FIGURE 3 instead of thedirect current amplifier. One can compare the embodiment of FIGURES 1and 2A and the embodiment of FIGURE 3. In the embodiment shown in FIGURE1 and FIGURE 2A, one collects all ions which have been given an energy(by the voltage pulse from circuit 40 applied to repeller plate 15 anddiscs 1 through 9) between the energies of V and V+AV times the chargeon the electron in each case. In the embodiment shown in FIGURE 3, theinstant invention collects all the ions which have been given an energygreater than V+ V sin wt. Since the value of sin wt varies with time,the minimum energy of the ions reaching the collector 30 in FIGURE 3varies with time and the output signal will have an alternating currentcomponent caused by ions with energies in the range from V-V to V+ V Thefrequency w/Z1r of the voltage applied to grid 20a of FIGURE 3, must besmall compared to the pulsing frequency of the voltage applied to therepeller plate 15. In both embodiments in order to vary the masscollected, one varies the voltage on grid 20a, that is, V. One does thisby varying the voltage continuously and slowly as one pulses the ionizermany times per second. As mentioned earlier, if the momentum given toeach ion is represented by P, M as the mass of an ion, and q as thecharge of an electron, which is considered to be substantially theamount of charge on each ion, then we have in the embodiment of FIGURES1 and 2A, using the approximation that AV is small compared with V, thestatement that the ions will be collected if they have an energy qVwhich is equal to P /2M. Similarly, in the embodiment shown in FIGURE 3we will get an alternating current signal on the collection plate 30 forthose ions for which the same equation applies, provided only that V issmall compared with V. Thus, as one scans the voltage with increasing V,one goes to smaller and smaller values of M, that is, the mass collectedis inversely proportional to the voltage applied at grid 200.

Since numerous changes may be made in the above apparatus and differentembodiments may be made without departing from the spirit thereof, it isintended that all matter contained in the foregoing descriptionreferring to apparatus or shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

We claim:

1. In a mass spectrometer, the combination comprising:

(a) means for providing pulses of ions;

(b) means for imposing a substantially constant momentum on the ions ina first direction, including a plurality of discs disposed substantiallyparallel to one another, said plurality of discs being connected bycircuit means to a source of potential, said plurality of discsincluding means defining a chamber through which the ions pass in afirst direction and said plurality of discs providing a uniform electricfield giving the ions constant momentum;

(c) first grid means for imposing a force on the ions in a directionsubstantially parallel and opposite to the first direction including adirect current voltage source; and

(d) second grid means for reflecting ions having energies within anarrow predetermined range and moving in the first direction, includinga direct current voltage source, said second grid means being sodisposed as to reflect only ions having energies within a narrowpredetermined range of those ions having sufiicient energy to passthrough said first grid means to provide a selection of ions on thebasis of their energy.

2. The mass spectrometer of claim 1 wherein said first grid means andsaid second grid means each includes a pair of grids positionedsubtantially parallel to one another and having means for imposing avoltage difference across said pair of grids.

3. The mass spectrometer of claim 2 including means for collecting theions and indicator means for indicating the current imposed by the ionimpingement upon said means for collecting the ions.

References Cited UNITED STATES PATENTS 2,769,093 10/1956 Hare et a1.25041.9 2,810,075 10/1957 Hall et al. 2504l.9 2,951,155 8/1960 Kindred250-419 RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

